1. Field of the Invention
This invention relates to an optical card reader, for use in a scanning receiver set with a PLL (phase-locked loop) frequency synthesizer, which is applicable to a system utilizing an optical card for presetting and memorizing a desired receiving channel.
2. Description of the Prior Art
As PLL (phase-locked loop) systems become popular with development in integrated circuits, change has been taken place from a conventional quartz oscillator control system to a PLL frequency synthesizer control system for a scanning receiver set in which a plurality of desired receiving channels are presetting predeterminedly and a scanning operation is effected on the preset channels until the scanning reaches any preset channel tuned to an allocated electromagnetic wave at which the scanning is stopped for receiving the electromagnetic corresponding to the tuned channel. In the synthesizer control system, a unit is needed for presetting and memorizing the desired channels. Various types of memory unit are available including a switch memory type, semiconductor memory type and card memory type, of which the card memory type is the most simple in view of the manufacture cost and easiness of operation because data can be written in a card by simply punching out holes in case of a card without hole or by blocking holes in case of a card with holes and many channels can easily preset by simply exchanging cards.
Typically, a conventional PLL frequency synthesizer comprises a programmable counter the frequency division ratio of which is determined by detecting the presence or absence of holes of card of the memory unit, a reference oscillator, a phase detector for detecting the phase difference between a reference signal from the reference oscillator and an output from the programmable counter to convert it into a variation in voltage, a low-pass filter for integrating and smoothing an output voltage from the phase detector, and a voltage controlled oscillator the oscillation frequency of which is controlled by receiving a control voltage of an output from the filter. The output of the voltage controlled oscillator is used, for example, as a local oscillation frequency of a radio receiver set. Such a PLL frequency synthesizer is well known as being disclosed in U.S. Pat. No. 3,937,972, for example.
This invention contemplates an improvement in the memory unit or optical card reader. Conventionally, with a card having a memory section of n channels and m bits (m and n being integers more than one), for example 16 channels and 16 bits, 256 (= 16 .times. 16) light emitting elements and 256 (= 16 .times. 16) light receiving elements are needed, resulting an extremely expensive unit. Where a single light emitting element is used for illuminating 16 bit illumination zones of respective channels in consideration of reducing the number of light emitting elements, it is necessary to realize the light emitting element as a tungsten lamp or the like of high illumination, suffering from short lifetime, generation of heat and so on. Further, with a light receiving element of CdS, when the scanning time of the receiver is set to about 80 m sec. for one channel, it is necessary to read out data within 20 m sec., since it takes about 60 m sec. for locking the PLL circuit and detecting electromagnetic waves. Light response speed of the CdS light receiving element is insufficient for this purpose.